Temporally-ordered CDW creep in pure and doped NbSe3

Below T=40 K, charge-density wave (CDW) transport in NbSe3 is characterized by two well-defined driving force thresholds ET and ET*. Between these thresholds the CDW moves extremely slowly with creep-like temperature and driving force dependencies. At the same time, the CDW exhibits coherent oscillations with a frequency proportional to the CDW current and having very narrow spectral widths, suggesting that the collective motion is temporally ordered. We have extended our initial work to doped crystals containing isoelectronic (Ta) and nonisoelectronic (Ti) impurities, and to crystals of different thicknesses. These experiments show that the qualitative features are extremely robust, and that the functional form of the creep velocity versus driving force and temperature is consistent across all samples for currents ranging over five orders of magnitude. The temperature dependence is consistent with processes having an energy comparable to the CDW gap, but the field and impurity dependencies are inconsistent with all predicted functional forms for creep in CDWs and related systems, and with our earlier picture of amplitude collapse at each impurity. We compare our results to measurements of creep-like behavior in other CDW and SDW systems, and discuss possible mechanisms.